Rosegarden -- it's just a combined MIDI sequencer and musical notation editor -- by itself it's nothing special. I have a few virtual MIDI input devices (one uses the mouse that I haven't tested, another uses the regular PC keyboard), but since I don't have a real MIDI keyboard, I have to write out notes by hand rather than record them.
Fluidsynth -- This is a realtime MIDI software synthesizer that lets you load whatever sound fonts you want and apply some various effects to them.
Jack-Rack -- This is just a LADSPA filter rack that you can plug the output of any audio program into using jack. If you plug a guitar into your line in, then connect one of your line in capture ports to jack-rack's input ports, plug jack-rack into your recording program, you can run some distortion and effects filters in realtime, as you're recording. If you also plug jack-rack to your line out, you can software monitor as you record too, without enabling any special features in your recording program. If you have multiple capture ports on your soundcard, you can even run each input through a completely different set of programs with different filters and effects.
Some other interesting Linux programs are hydrogen for creating drum beat loops and ardour for recording audio tracks. You can also use
ZynAddSubFX to make all sorts of tweaked, custom instruments, but I haven't had a chance to figure that one out yet.
I'm just starting to scratch the surface with this stuff though. There's so many little programs and tools out there for Linux audio that it's like being a kid in a candy store -- and I haven't even developed a good taste for candy yet. Except in this case, the candy is all free.
Not to mention that as a programmer, it's even more fun, since I can create my own effects and add my own software to the workflow very easily, due to the fact that Jack's C API is really simple and straightforward.